The present invention relates to a method and apparatus for resecting the distal surface of a human femur to prepare that surface to accept a distal femoral prosthesis and, more particularly, to a method and apparatus for making the initial resection along the transverse axis of the knee joint which is perpendicular to the mechanical axis of the leg.
There are many types of distal femoral knee prostheses currently in use, and in many instances, a specific femoral knee prosthesis utilizes a specially designed jig which enables a surgeon to make the necessary resections to prepare the distal femoral surface to accept the prosthesis. The ultimate goal with any knee prosthesis is to approximate the natural, healthy condyles which the prosthesis is replacing. Should the prosthesis not be properly attached to the femur, the resulting misalignment could result in subsequent discomfort to the patient, walking problems, or degradation of the prosthesis.
In attaching a knee prosthesis, it is desirable to orient the prosthesis such that the transverse axis--the pivot axis of the knee joint--lies within a transverse plane which is perpendicular to the mechanical axis of the femur. The mechanical axis lies along a line which intersects the femoral head and the center of the ankle. The mechanical axis can be determined from an inspection of a radiograph of the femur to be resected. During the actual resection operation, the mechanical axis is determined by computing its angle--the valgus angle--from the femoral shaft axis. Accordingly, it is necessary to align any cutting jig or fixture properly with respect to the femoral shaft axis.
There are many jigs designed to be attached to the femur which include structure for accurately aligning the jig with the femoral shaft axis. Characteristic of such a device is the Laskin Precision Total Knee Instrumentation Assembly, manufactured by Richards Medical Company, and disclosed in the catalog entitled Total Knee Replacement with the Tricon-M System, published by Richards Medical Company, 1984.
The jig disclosed in that publication includes a femoral intramedullary stem which is inserted through the intercondylar notch and upwardly through the femur along the femoral shaft axis. The stem includes a bracket which supports a distal femur cutting guide. The bracket includes a first pin which extends through the cutting guide and acts as a pivot axis. A second pin is attached to the bracket and extends through an arcuate slot in the cutting guide and receives a wing nut. A boss is also attached to the bracket and extends through a second arcuate slot in the cutting guide. The second slot includes markings indicating the angle the cutting guide is pivoted from alignment with the femoral shaft axis.
The cutting guide includes pairs of opposing slots formed along its sides which are oriented to be perpendicular to a central axis of symmetry of the cutting guide. When the cutting guide is pivoted such that the central axis of symmetry lies along the mechanical axis--thereby forming the appropriate angle with the femoral shaft axis--these cutting guide slots are positioned to be perpendicular to the mechanical axis. The cutting guide is locked into the predetermined angle with the femoral shaft axis by tightening the wing nut against the upper surface of the cutting guide, which clamps it against the bracket.
The depth of the initial resection is determined by the dimensions of the particular femoral implant to be utilized. For example, a femoral implant manufactured by Richards Medical Company has a distal thickness of approximately 1.2 cm. Accordingly, the resection should usually be made a distance of 1.2 cm from the distal condylar apices. In order to position the cutting guide an appropriate distance from the distal portion of the femur, the aforementioned jig includes a resection guide, which is a flat plate that attaches to the rear or distal portion of the cutting guide.
To attach the Laskin Assembly to a femur, first the intramedullary stem is displaced into the femur until the resection guide contacts one or both the condylar apices. The jig is then secured in position during the cut by bone spikes which are inserted through openings in the cutting guide and into the femur. The flat surface resulting from such a cut, which lies along the transverse axis, serves as a reference surface from which other resections, such as the anterior and posterior resections, are made. Accordingly, it is necessary for this initial resection to be made with a high degree of precision.
A disadvantage with the aforementioned jig is that the cutting guide is held in its postion along the mechanical axis by the clamping engagement of the wing nut, so that the cutting guide may move slightly from its desired position as a result of vibrations received from the moving blade of the resecting instrument, typically an electric saw.
Other types of resecting guides utilize a more positive mechanism for positioning the saw guides. For example, Lacey U.S. Pat. No. 4,502,483 discloses an apparatus for resecting a distal femoral surface which includes a main body having a first plurality of holes and a guide holder which is pivotally attached to the main body and includes a second plurality of holes. The pluralities of holes are arranged such that pairs of holes from each of the pluralities of holes come into registry with each other at predetermined degrees of inclination of the holder relative to the main body. A screw is inserted into the pair of holes in registry to lock the holder relative to the main body. The main body is attached to the femur by a clamp.
A disadvantage with both of the aforementioned types of devices is that they are not designed to compensate for severe femoral condyle deficiency, wherein one condyle is reduced in size relative to the other. With those devices, there would be a tendency to remove an insufficient amount of bone from the deficient condyle. Although the Laskin device contains multiple blade slots for varying the depth of cut, the depth of cut can be varied only on a large scale and only in predetermined increments. The Lacey device lacks altogether a mechanism for varying the depth of cut.
Another disadvantage of the aforementioned devices is that they are specifically adapted to the shapes of a particular joint prosthesis, so that they may not be used to perform resections to prepare the femur for different types of prostheses.
Accordingly, there is a need for a method and apparatus for resecting a distal femoral surface which is sufficiently flexible to perform a resection in cases involving a severe femoral condylar deficiency. Furthermore, there is a need for a method and apparatus for resecting a distal femoral surface to accommodate different types of femoral prostheses.